Originally Posted by Doru

The OP is familiar with BMW's. He's not familiar with e53.

Hence, there are a few common tell-tale signs for specific e53 models, that even after an inspection don't show up, but they show after you drive the vehicle.

I will try do make some of these points (stuff that is considered "common"), maybe some others can chime in. By model would be great.

2004-2006 e53 N62:

• Assuming visual inspection checks out, incl under the hood, see how "tight" it feels when opening the hood. If it feels "tight", the cables could be rusted, on the way out. Easy & cheap fix, to be done ASAP, so you don't end up busting the grille to access cables. This is valid for ALL e53 models, I believe.
• if the rear wheels seem to have a high camber, especially with wheels considered "small" (17" - 18"), probably the rear ball joints are toast. The only way to check, is one side of the car in the air, and deflate the air bag, otherwise it will put pressure on the suspension component, and it will feel tight. Once the air is out, if the ball joint is toast, the wheel will wiggle.
• Rear of transmission - check for oil leak. if it leaks, the sealing sleeve is toast. This part has been replaced with an updated part, that has black O-rings (the original has red ones), and will not leak anymore. Having low ATF level will induce a lag & a hard shift felt as a "clunk" when slowing down, the trying to accelerate, or when going from reverse to drive. Was on my N62, not sure about previous models
• Once the engine is warmed up to operating temp, idling for a few minutes, then revving the engine - if it puffs a blue/black cloud of smoke - usually a bunch of seals & gaskets are gone and the debatable valve stem seals as well. N62 engine
• This one, not sure how explain: If the car sits in the cold after it was driven (not garaged, or in non-heated garage) and there is coolant drip: pending from where it drips: a.) could be the lower radiator hose (very common) or b.) the cooling pipe starts coming apart. Has to be assessed carefully. N62 engine.
• If the car has more than 100k miles, and the engine rattles, could be chain guides. M62 engine.
• The M54 engine to test for issues, prior to starting it up, make sure the engine is stone cold. Most issues will unveil once you start it up. A warm engine can be started multiple times, no ill signs will show up.
• On all models, the front CV boots can go - this should be a mandatory pre-purchase inspection. It's a normal wear & tear item, but can get expensive if not caught in time.
• Rear air bags (not sure how long they actually last) - on my car I caught them "red handed" after the car sat outside in -20°C (-4°F) - I was visiting friends, and when I left, I could hear the compressor going steady. This didn't happen with the car garaged. Situation repeated throughout the winter when I had to park the car in the cold. New bags solved the issue (Arnott). Easy and relatively cheap fix.
• If living in a cold environment, CCV on M54 need TLC, if equipped with OE system. Should be replaced with cold weather CCV including re-designed dipstick (otherwise it's pointless).
• Some irritating rear view mirror issues (easy DYI fixes), pixilation of the cluster and HU (can be repaired for good)
• Seems like the M62 & M54 engines are more susceptible to cooling issues than the N62 counterpart - I mean the rubber hoses, expansion tank, valley pan gasket, although the N62 has some other voes mentioned above.

I am sure there might be other things, but I believe these are the most frequent/mentioned over the years.
I also believe that these cars are quite reliable if the owner has a scheduled maintenance record of the car, sticks to it, and checks it out periodically for unforeseen issues that can arise on any model/make of a used vehicle.
I have no issue with my cars, and I own 3 different models BMW, each with it's own specifics, but as I said, in the end I'm very happy how they perform.